Introduction
The number of mobile phone users has hugely increased in the last 2 decades. Since the phones are usually held close to the ear, possible adverse effects on the hearing system are of concern. Most studies on this subject deal with effects of short-term exposures (10 min to 1 hour). Information on the effects of long-term mobile phone exposures is scarce. No studies evaluated effects of code division multiple access (CDMA) mobile phones on hearing.

Objective
The objective of this study was “to assess and compare potential changes in hearing function at the level of the inner ear and central auditory pathway due to chronic exposure to electromagnetic waves from both global system for mobile communications (GSM) and code division multiple access (CDMA) mobile phone usage”.

Methods
Participants were 125 long-term (>1 year) mobile phone users aged 18-45 years and 58 control subjects who never used mobile phones. Sixty-three of the participants used GSM phones and 62 used CDMA phones. The study subjects had no external or middle ear pathology and no history of medical conditions or exposures that could affect hearing. Investigations included pure tone audiometry, speech discrimination score, speech reception threshold, impedance audiometry, distortion product otoacoustic emissions (DPOAE), auditory brain responses (ABR) and middle latency responses (MLR). These examinations were performed for the “mobile phone-using ear”, “non-mobile phone-using ear” and the corresponding ears of the control subjects.

Results
Compared to the control subjects, the GSM phone users had significantly higher speech frequency thresholds, lower MLR wave Na amplitude and wave Pa amplitude, and a higher risk of DPOAE absence in the mobile phone using ear. The CDMA phone users had significantly lower MLR wave Na amplitude in the mobile-phone using ear and a higher risk of DPOAE absence if the mobile phone using ear was the left ear. The audiological parameters did not differ significantly between the GSM and CDMA phone users or between the mobile phone using ear and the non-mobile phone using ear. Subjects who used mobile phones for more than 3 years had lower MRL Na wave amplitude (CDMA users) and a higher risk of DPOAE absence (GSM users) than mobile phone users of shorter duration.

Interpretation and Conclusion
DPOAE is an indicator of cochlear functionality. Changes in amplitudes of MRL waves Na and Pa may reflect disturbances in the auditory cortex. The results of this study suggest that mobile phone users may have bilateral subclinical hearing loss. The authors discuss limitations of their study, such as small size of the study group and potential confounding from environmental noise or presence of a base station in the vicinity.